I would agree with what most people have said here (my family has a sawmill...3 if you count the shinglemill and chainsaw sawmill). That being said, for the money, I find its easier to hire a sawyer to come in and saw my logs.
Size depends on the tree species. For hardwood trees, I don't cut anything smaller then a foot in diameter. For well paying trees like Ash, Oak, Spruce, Hemlock and Yellow Birch, I won't cut anything less then 18 inches in diameter. For White Pine, nothing under 36 inches in diameter. The smallest diameter I cut on the small end of any log, is 6 inches.
Most sawyers in my area typically charge 180 a thousand for sawing...if you got decent wood. If you cut "fence posts" (small logs) they will often charge you a lot more for the inconvenience. They also like to have a 1500 bf minimum. Sure they will saw only 1000 bf for you, but you still get charged for 1500 ft. You might want to ask your sawyer. Also ask him how he wants the logs piled. My sawyer prefers logs on flat ground, with plenty of room to move his mill/truck around, and on a hard surface like a gravel road or someplace he won't get stuck. Since he moves the logs by hand, he likes to have the pile less then 4 feet high. He also likes all the logs in one pile so he does not have to move his sawmill set up once he's up and running.
So why would you care? Well trust me, the guy sawing your lumber can do a good job, or do a bad job. The easier you make it for him, the better the wood comes out.
There is way, way more on this subject, but way to much to type here. Please take the time to checkout my website. I have a whole section dedicated to a virtual harvest. What I did was go through the process of cutting wood from stump to sawmill. The pictures are small (dialup days) but the text describes in detail how to get the most out of your small harvest. I think you can get a lot of information out of this. The research and questions you invest in now, BEFORE you start your chainsaw, will pay you great dividends AFTER the trees are felled.
I wish you the best of luck though, and watch out for those chainsaws. I got bit 2 weeks ago pretty good.
I have no intention of traveling from birth to the grave in a manicured and well preserved body; but rather I will skid in sideways, totally beat up, completely worn out, utterly exhausted and jump off my tractor and loudly yell, "Wow, this is what it took to feed a nation!"